Function pad_spec 

fn pad_spec(
    dims: (usize, usize, usize),
    dec_len: usize,
) -> ((usize, usize, usize), usize)

Padded dimensions and decomposition level, matching WaveSep’s pad_spec: grow each axis to a common multiple P (starting at 16) until every padded dim is divisible by 2^L, where L = dwt_max_level(min_padded, dec_len).